In any system which uses a compressor for compressing refrigerant, there is some form of control apparatus to energize and deenergize the compressor at appropriate times. This control apparatus can take various forms from the simplest configuration involving little more than a thermostat and a relay to somewhat more sophisticated systems involving multiple relays or, more recently, control devices with programmable microcomputers. Whatever the level of complexity, the last component between the power lines and the compressor is a relay, either electromagnetic or solid state.
With an electromagnetic relay, it is well known that a condition can occur known as welded contact failure. This phenomenon can arise when a current surge occurs as the contacts of the relay are opening. Sufficient heat can be generated to melt the contacts themselves, causing them literally to be welded together in their closed condition. Obviously, when this occurs, the relay has lost all control over the operation of the load being controlled, in this case a compressor, and the compressor continues to run regardless of need. Commonly, there is no load on the compressor after the contacts are welded so the compressor runs itself to destruction unless there are safety devices used. This kind of failure is referred to by the traditional term "welded contact" even if the control system is entirely solid state and, strictly speaking, has no contacts to weld. When it occurs, the nature of the failure in a solid state relay is similar to that in a mechanical relay in that a very low resistance short circuit develops through the solid state relay, forming an uncontrolled path for power to the compressor.
Destruction of a compressor under these conditions can be a catastrophic event. The pressures and temperatures in the compressor are likely to be quite high. Thus, when the machine fails, the result can be an explosion which is dangerous to people in the vicinity as well as to other equipment. For this reason, it has been common to build some form of safety device into the system, such as a ball check valve built into the housing of the compressor itself to bypass the fluid flow and limit the pressure differential which can develop. While this protects against a dangerous explosion, it does not save the compressor which is allowed to continue running and is usually not usable thereafter.
Another form of safety device is a circuit breaker connected to open all of the power lines to the compressor motor in response to excessively high pressure or temperature or high current. While this kind of device is effective, it is very expensive and obviously increases the total cost of the system in which it is employed.